1. Field of the Invention
The present invention relates to an electrical capacitance proximity sensor for detecting a nearby object in response to a variation in electrostatic capacitance and, more specifically, an electrical capacitance proximity sensor having a self-diagnostic function.
Priority is claimed from Japanese Patent Application No. 2004-208341, filed Jul. 15, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
An electrical capacitance proximity sensor for detecting a nearby object is well known in the art. Such an electrical capacitance proximity sensor converts a variation in electrostatic capacitance between a detecting electrode and a ground electrode caused by approaching the nearby object into a variation in an oscillation frequency, transforms or linearizes the oscillation frequency into a direct current voltage, and compares the direct current voltage with a predetermined threshold value to detect the nearby object. A separate structure has been proposed for the electrical capacitance proximity sensor in which a sensor section which includes the detecting electrode and the ground electrode is apart from a detecting circuit section which includes an oscillation circuit and a comparator. In this separate structure, an electrostatic capacitance is inevitably generated in a cable which connects the sensor section and the detecting circuit section and influences the variation in the electrostatic capacitance detected by the sensor section as an error, and an accurate detection of the nearby object is prevented.
To solve this problem, an electrical capacitance proximity sensor is proposed in Japanese Unexamined Patent Application, First Publication No. H07-29467 (see paragraphs 0006 through 0008 and FIG. 1). In the electrical capacitance proximity sensor, a shield electrode is interposed between the detecting electrode and the ground electrode so as to be insulated from both the detecting and ground electrode. The detecting electrode and the shield electrode are connected to a core wire and a coated wire of a shield cable, respectively. On the detecting circuit side, the core wire and the coated wire are connected to input and output terminals of a buffer circuit, respectively, in which the detecting electrode and the shield electrode are always held at the same phase and the same voltage. According to this structure, the core wire and the coated wire of the shield cable are also held at the same phase and same voltage so that no charge or discharge between the core and coated wire occurs. As a result, the above structure prevents the detecting circuit section from being influenced by the electrostatic capacitance generated in the cable.
Although the sensitivity of detection is improved in the electrical capacitance proximity sensor of the related art as described above, it does not operate well and cannot see the reason for malfunction cannot be ascertained when the sensor section and the detecting circuit section are incorrectly connected.